Senescence is a tool of embryonic development repurposed to wound healing and cancer suppression. Cells need to be suppressed once the finger is done being built at the end and the edges, and senescent cells do that job. Cancerous tissue needs to be suppressed, and senescent cells are there to do that job.

The problem is not that senescent cells exist, it is that there is not 100% coverage of their removal after they do whatever the short-term job at hand happens to be. It is the lingering ones that cause the harm.